Vein authentication is known to be one of the modes of the biometrics authentication technology. While performing vein authentication, a palm of the subject to be authenticated is irradiated with infrared light, and the reflected infrared light from the surface of the palm or from the inside of the palm is captured using a camera.
In the case of implementing the reflection-type imaging technique for capturing the veins; not only the diffuse reflection light that captures information about the veins falls on the lens of the camera, but also the specular reflection light that captures information about the surface of the palm also falls on the lens of the camera.
FIG. 7 is a diagram for explaining diffuse reflection. In the example illustrated in FIG. 7, arrows represent the light paths of the infrared light emitted from a light source. As illustrated in FIG. 7, as far as the diffuse reflection light is concerned, the infrared light emitted from the light source penetrates through a palm 71 and diffuses in repeating fashion before coming out to the surface of the palm 71. At that time, the angle of reflection of the infrared light is not dependent on the angle of incidence; and the light falling on the inside of the palm 71 is uniformly reflected in all directions. Regarding that diffuse reflection light, the light of only a specific wavelength is absorbed by the reduced hemoglobin present in the blood. As a result, only from the portion covered by a vein 72, the light comes back at a weaker intensity as compared to the intensity at the time of emission. For that reason, in an image in which the diffuse reflection light is captured, the vein 72 appears dark. In this way, as a result of capturing the diffuse reflection light, the vascular pattern of veins becomes visible.
FIG. 8 is a diagram for explaining specular reflection. In the example illustrated in FIG. 8 too, arrows represent the light path of the infrared light emitted from a light source. As illustrated in FIG. 8, as far as the specular reflection light is concerned, the infrared light emitted from the light source is reflected from the surface of the palm 71 and comes back. At that time, since the direction of the light path is preserved due to surface reflection, an angle of incidence θ1 and an angle of reflection θ2 are identical to each other. However, the information about the inside of the palm 71 is not included in this surface reflection. Not only that, the patterns such as the wrinkles present on the surface of the palm 71 are captured, thereby obscuring the information about the veins that is obtained due to the diffuse reflection light. Sometimes, such surface reflection causes interference to the authentication.
Exemplary methods for reducing the effect of such surface reflection are as follows. For example, in a first method, the light source is placed at a sufficiently distant position from the camera with the aim of increasing the angle of reflection, and the specular reflection light is prevented from falling on the camera. In a second method, polarized light is emitted, and a polarization filter is disposed in front of the camera with the aim of cutting polarization in the same direction as the direction of polarized light. With that, the specular reflection light is removed. In a third method, a plurality of images is taken while changing the illumination; and the reflection-free portions in the images are synthesized. In a fourth method, a low-pass filter is applied to endoscopic images, and filtered reflection images are obtained. These related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2002-112970, Japanese Laid-open Patent Publication No. 2002-200050, Japanese National Publication of international Patent Application No. 2002-501265, Japanese Laid-open Patent Publication No. 2002-133446, Japanese National Publication of international Patent Application No. 2002-514098, and Japanese Laid-open Patent Publication No. 2001-224549
However, in the conventional technology described above, in order to reduce the effect of surface reflection, the device scale or the manufacturing cost gets sacrificed as explained below.
For example, in the first method, there is a limitation to increasing the distance between the position of the camera and the position of the light source. Hence, by increasing the installation area of the device, the device scale becomes bloated. In the second method, since a polarization filter needs to be disposed in the camera, there occurs an increase in the manufacturing cost. Not only that, because of the polarization filter, there occurs an increase in the attenuation rate of the light falling on the camera. As a result, the image quality undergoes deterioration or the power consumption of the illumination goes higher. In the third method, a plurality of illuminations or a plurality of camera are used. That leads to an increase in the manufacturing cost and bloating of the device scale. Besides, the imaging time of images becomes lengthy too. In the fourth embodiment, the low-pass filter is usable only for endoscopic images and does nothing more than performing filtering of reflected images of the illumination that appears on a smooth surface such as that of internal organs. Hence, such a low-pass filter is not applicable to images that capture a palm having a complex diffusing surface.
In a vein authentication method laid open in this application, a computer executes an operation of converting pixel values of an image, which captures an authentication site including veins, into frequency components. Moreover, in the vein authentication method, the computer performs an operation of filtering the frequency components, which are obtained by conversion of the pixel values, using a filter stored in a filter storing unit which is used to store a filter for reducing frequency components, from among low-frequency components having a lower spatial frequency than the spatial frequency of the veins and high-frequency components having a higher spatial frequency than the spatial frequency of the veins, corresponding to surface reflection of an illumination used for the authentication site. Furthermore, in the vein authentication method, the computer performs an operation of converting the frequency components, which are subjected to filtering, back into an image. Moreover, in the vein authentication method, the computer performs an operation of extracting vein data, which represents a vascular pattern of veins, from the image obtained by reverse conversion. Furthermore, in the vein authentication method, the computer performs an operation of performing vein authentication using the vein data that is extracted.